WASHINGTON – FBI agents still shudder when they recall Feb. 7, 1995 – the day they brought fugitive master terrorist Ramzi Yousef back to New York to face justice.

As a helicopter circled Manhattan carrying the man who masterminded the 1993 bombing of the World Trade Center and plotted bombings of New York landmarks and 11 American airplanes, an FBI agent pulled off Yousef’s blindfold to show him the Twin Towers were still standing.

“They wouldn’t be if I had enough money and explosives,” Yousef is said to have told him.

Almost seven years later, Yousef’s cataclysmic dream became an astonishing reality. The attack last Tuesday was the worst ever on American soil.

Hell rained down from the skies over New York and Washington. Two hijacked flights from Boston slammed into the Trade Center, killing thousands of people, leveling the New York landmark and changing American lives forever.

Less than an hour later, a third hijacked aircraft, from Washington’s Dulles Airport, threatened the White House before descending into a fireball at the Pentagon.

A fourth hijacked aircraft, from Newark, crashed into a Pennsylvania field, having failed to reach its intended target, Air Force One, which was due to land at Andrews Air Force base east of Washington with President Bush on board.

It was a terrorism spectacular like no other the world has ever seen – audacious and brilliant in its conception, disciplined and diabolical in its execution.

And it caught a superpower, with all its space-age spy satellites and high-tech military might, completely by surprise.

While Yousef and other plotters of the first Trade Center bombing languish in federal prisons, it soon became clear to FBI investigators in the awful aftermath that Tuesday’s kamikaze attacks were carried out by fellow “Afghan Arabs” who have the same support networks, follow the same fanatical religious ideology of jailed Sheik Abdul Ahmed Rahman, and follow the deadly orders of Osama bin Laden.

“This is something that started back in 1993 with the first World Trade Center bombing. But we didn’t get everyone that time, and the people we didn’t get continued to scheme and plan, and they got more sophisticated,” said Kenneth Katzman, a former CIA analyst.

From what the FBI investigation, code-named PENTTBOM, has been able to piece together so far, it is clear that bin Laden’s Al-Qaeda organization made sure that perpetrators of Tuesday’s Pentagon attacks had what Yousef lacked back in 1993 – firepower, money, preparation and discipline – for their mission of mayhem.

Years of planning

The FBI believes planning for Tuesday’s attacks began five years ago. Preparations were so meticulous that FBI agents and U.S. intelligence analysts concede that what they know already has left them astonished.

The plot spans three continents – from bin Laden’s base camps in mountainous Afghanistan to safe houses in Hamburg, Germany, and Montreal to quiet communities and unsuspecting flight schools in Florida, where the hijackers prepared for their apocalypse.

The plot also includes reports of stolen pilots’ uniforms in Rome, “sleeper” terrorist agents at airports, forged runway passes at Boston’s Logan Airport, and a meticulous study of the power of airline fuel.

The plotters checked out security procedures and flight schedules. They selected big jets, headed cross-country, because they were loaded with fuel to assure maximum devastation of their targets.

Intelligence sources say they believe Al-Qaeda operatives even tested U.S. intelligence capabilities last June in a plot straight out of a John LeCarre novel.

CIA and National Security Agency officials believe that information leading to a worldwide terrorist alert that forced U.S. Navy ships in the Persian Gulf to leave their ports was disinformation planted by bin Laden’s operatives to test what the U.S. knows – and doesn’t know – about how they operate.

“This was a brainy bunch,” said former CIA counterintelligence officer Paul Redmond.

The FBI still doesn’t know the identity of the operational commander of Tuesday’s horrific attacks – the mastermind who activated the “sleeper” agents and put together the cells that carried out the attacks.

But more is being learned every day about the 19 men picked for the assignment. The FBI believes they were all Egyptians or Saudis, members of the Islamic Jihad and al-Gamaa al-Islamiya, who form the core of bin Laden’s most trusted Praetorian Guard in the amorphous, globally based Al-Qaeda.

They were elite, disciplined, well educated, and so dedicated to bin Laden’s cause of ridding Muslim lands of Western influence that they were willing to die for their mission.

One, Waleed Shehri, was the son of a wealthy Saudi diplomat in the United States.

Another, Marwan al-Shehhi, 23, was a student at a local technical university in Hamburg.

Others were veteran terrorists. Two were on a special terrorist-watch list given to INS agents and border patrols.

Mohamed Atta, 33, was a student who took part in the bombing of a bus in Israel in 1996, according to law-enforcement officials.

Seven of the hijack bombers had pilot’s licenses. Others had received lesser forms of aviation training and extensive training in hand-to-hand combat, disguise, counterintelligence and other deadly arts needed for their mission.

The first member of bin Laden’s hit teams, Waleed Shehri, entered the country quietly in 1996 and studied at the Embry-Riddle Aeronautics University in Daytona Beach, Fla.

Over the next five years, other suicide pilots began arriving and quietly burrowed themselves in quiet communities, among them Hollywood, Fla.; Phoenix; Fort Lee, N.J., Vero Beach, Fla., and Los Angeles. Five lived in Delray Beach, Fla.

U.S. military training

In the most astonishing discovery so far, two of the hijackers attended U.S. military war schools – one at the Air War College in Alabama, the other at the Defense Language School at Lackland Air Force Base in Texas.

Some frequented local bars and shops and let their kids play in their neighborhoods. Two attended community colleges in the San Diego area.

Others left no trace of their existence. They tapped into a shadow network of bin Laden operatives already active in the United States, who helped them find places to live and gave them money and false identity papers.

The terrorists believed they would die, become martyrs and live in paradise. But they did not know their mission at the time or who else was involved.

Law-enforcement officials believe the terrorists waited for someone to come to the United States and activate them on behalf of the jihad.

Atta and Shehhi, believed to have been cousins, received pilot training at the Huffman Aviation Center and jet-cockpit simulator training at the SimCenter Inc. in Opa-Locka, Fla., where they said they were planning to fly for a rich Arab.

People who met them remember they were quiet, polite, enthusiastic students, talked a lot on cell phones and were always flush with cash.

Police believe Atta, considered the most experienced of the group, began casing Boston’s Logan Airport about six months ago. He was last seen on Sept. 9 in Hollywood, Fla., returning a rental car and, later that night, in a bar where he arrogantly pulled out a wad of cash.

Police theorize that Atta, the chief hijacker of American Airlines Flight 11, hooked up with four members of his hijack team in Portland, Maine, the night before the attacks.

His team included Shehri, who stayed in a hotel in Newton, Mass., where flight-training manuals were found.

With Shehri, also a graduate of the Embry-Riddle Aeronautical University, were Satam Al-Suqami and Abdul Alomari, who lived in Vero Beach, Fla., with his wife and four children and moved out of his $1,400-a-month rental home on Sept. 3.

Suqami is believed to have spent time in Canada in the days before the mission.

A second team, which seized United Airlines Flight 175, was headed by Shehhi. The cell lived quietly in Delray Beach, and included Fayez Ahmed, Ahmed Alghamdi, Hamza Alghamdi and Mohaid Al-Sheri.

Many members of this unit are believed to have hooked up in the Boston area, where police have traced credit-card bills.

In all, $50,000 worth of airline tickets were purchased in advance, according to the FBI. Reservations for seven of the hijackers were paid with the same credit card, sources said.

The team that hijacked American Airlines Flight 77 appears to have been headed by Hani Hanjour, believed to have been a Saudi citizen who received an FAA pilot’s license in 1999.

Other hijackers of that flight have been identified as Khalid Al-Midhar, a resident of New York and Los Angeles, as well as Majed Moqed and Nawaq and Salem Al-Hamzi, both of Fort Lee, N.J. The Justice Department identified the fourth team, which hijacked United Airlines Flight 93, as Ziad Jarrahi, who possessed a Hamburg, Germany, pilot’s license, and Saeed Al-Ghamdi, Ahmed Al-Nami and Ahmed Al-Haznawi, all of Delray Beach.

Final journey

On the morning of Sept. 11, Atta’s Portland team missed a 5:30 a.m. flight and drove to Boston in a rental car, where they got into a fight with another motorist over a parking space and almost missed the suicide flight, according to Boston cops.

But, tragically, the mission went off with deadly precision.

Flight 11 left Boston with 92 people aboard at 7:45 a.m. bound for Los Angeles. At some point during the flight, Atta’s team, armed with knives, seized control of the aircraft and flight crew and headed to New York, where the plane, loaded with fuel, crashed into the north tower of Trade Center at 8:45 a.m.

Flight 175 with 65 people aboard left Boston bound for Los Angeles at 7:59, and, while flying over upstate New York, suddenly veered southward toward New York City and, with TV cameras trained on the emerging tragedy, hit the south tower at 9:05 a.m.

The third team got control of Los Angeles-bound American Airlines Flight 77 with knives and boxcutters an hour after it departed Dulles Airport near Washington.

Air controllers lost touch with the plane after someone, apparently one of the hijackers, turned off the plane’s transponder.

Passenger and CNN commentator Barbara Olson twice called her husband, U.S. Solicitor General Theodore Olson, on her cell phone, reporting that she and fellow travelers had been herded into the back of the jet and appeared to be headed on a death mission to the White House.

The FAA notified the White House of the impending attack, and two F-16 jet fighters were scrambled from Langley Air Force Base. But just as the jet seemed on the verge of completing its mission, it swerved suddenly and seconds later slammed into the Pentagon, carving a gaping hole in the defense headquarters and killing 190 military employees inside.

The day of shock was still not over. San Francisco-bound United Airlines Flight 93 out of Newark with 45 people aboard lifted off at 8:01 a.m. Sometime near Cleveland, the flight made a U-turn and headed back toward Washington.

From cell-phone calls from passengers and conversations in the cockpit, federal authorities learned the intended target of the suicide pilots was Andrews Air Force Base.

President Bush, in Sarasota, Fla., was due to land at around the same time Flight 93 would reach its destination.

Authorities believe the passengers may have tried to overpower the hijackers, causing the flight to crash in Stony Creek Township, Pa., at 10:10 a.m.

A few hours later, the National Security Agency intercepted a cell phone call from the United States to a suspected bin Laden operative in Europe – “We hit the targets,” was the message, sources said.

Manhattan war zone

The attacks turned lower Manhattan into a war zone, rocked the government and traumatized the nation in a way not seen since the Japanese attack on Pearl Harbor.

But as 7,000 FBI agents fanned out across the country looking for the co-conspirators, some disturbing information was developed, leading U.S. officials believe that there may be more nasty surprises to come from bin Laden and his disciples in the days and weeks to come.

“We have reviewed the [intelligence] data since the bombing, and we don’t really like what we are seeing,” a senior U.S. defense official told The Post.

Ahmed Ressam, the Algerian national caught at the U.S.-Canadian border in December 1999 on his way to help bomb Los Angeles International Airport, has given a chilling warning of our country’s enemy.

Ressam recently told authorities – as part of a plea agreement – that he received special training in bin Laden’s camps in Afghanistan on urban warfare, assassination and sabotage, and took a special course in “how to blow up the infrastructure of a country,” according to court documents.

Ressam also said he believes there are several bin Laden terror cells in this country, all operating independently of each other and given wide berth to chose their own targets.

On the morning of Sept. 11, as this grave national calamity was unfolding, a group of suspected Islamic militants gathered on a rooftop in Union City, N.J., to videotape the collapse of the Twin Towers just as Ramzi Yousef had eight years earlier, according to information given to law-enforcement authorities.

When the doomed jets hit the gleaming superstructures, the gang reportedly cheered. They are among the many people now being hunted by the FBI.